1. Field of the Invention
The present invention relates to media used in drug elution studies and methods for making and using them.
2. Description of Related Art
The implantation of a medical device into a patient's body can cause the body to exhibit adverse physiological reactions ranging from infections to the formation of emboli or clots in blood vessels. One approach to address such reactions and improve the biocompatibility of such medical devices is to incorporate bioactive or pharmacological agents such as steroids, and/or antibiotics and/or anticoagulants onto a surface of these devices. Once implanted, these agents can then elute into the in vivo environment at the site of implantation and modify the physiological response.
Exemplary medical procedures that involve the implantation of medical devices include those designed to modulate cardiac physiology. For example, a variety of systems that use one or more pacing leads with electrodes such as cardiac rhythm management (CRM) systems and various techniques for implanting these lead systems in contact body tissue such as the heart, have been developed. In this context, the safety, efficacy and longevity of an electrical pulse generator of a CRM depends, in part, on the performance of the associated cardiac lead(s) used in conjunction with the pulse generator. For example, various properties of the lead and electrodes will result in a characteristic impedance and stimulation threshold. Stimulation threshold is the energy required in a stimulation pulse to depolarize, or “capture,” the heart tissue. A relatively high impedance and low threshold is desired to minimize the current drawn from a pulse generator battery in delivering a stimulation pulse. Another example may be when implanting a medical device, it can result in infections in the area of infections. Such infections can be reduced by coating or otherwise combining an antibiotic with the implanted device.
One factor that can affect the stimulation threshold, particularly during the first several weeks after implantation of a lead, is the natural immunological response of the body to the lead as a foreign object. The presence of the lead activates macrophages, which attach themselves to the surface of the lead and any electrodes and form multi-nucleated giant cells. These cells, in turn, secrete various substances, such as hydrogen peroxide as well as various enzymes, in an effort to dissolve the foreign object. Such substances, while intending to dissolve the foreign object, also inflict damage to the surrounding tissue. When the surrounding tissue is the myocardium, these substance cause necrosis. These areas of necrosis, in turn, impair the electrical characteristics of the electrode-tissue interface. Consequently pacing thresholds rise. Even after the microscopic areas of tissue die the inflammatory response continues and approximately seven days after implant the multi-nucleated giant cells cause fibroblasts to begin laying down collagen to replace the necrotic myocardium. Eventually, on the order of three weeks after implant, the lead and its electrodes can be encapsulated by a thick layer of fibrotic tissue. Typically, the inflammatory response ends at this time. The fibrotic encapsulation of the lead and its electrodes, however, remains. Since the fibrotic tissue is not excitable tissue, an elevated stimulation threshold can persist due to the degraded electrical properties of the electrode-tissue interface.
One means of modulating this inflammatory response in implanted cardiac rhythm management systems has been to provide a drug near the electronic lead to mitigate the inflammatory tissue reaction described above. In particular, it has been found devices designed to elute an anti-inflammatory agent, such as a glucocorticoid steroid, minimize tissue irritation, help reduce or eliminate threshold peaking and further assist in maintaining low acute and chronic pacing thresholds. A considerable breakthrough in the development of low threshold electrode technology occurred with the invention of the steroid eluting pacing electrode of Stokes U.S. Pat. No. 4,506,680 and related Medtronic U.S. Pat. Nos. 4,577,642, and 4,606,118. Steroid, it is believed, inhibits the inflammatory response by inhibiting the activation of the macrophages. Because they do not form multi-nucleated giant cells, the subsequent release of substances to dissolve the object and which also destroy the surrounding tissue is prevented. Thus, the necrosis of any tissue by the inflammatory response is minimized as well as the formation of the fibrotic capsule. Minimizing such adverse reactions is highly desirable because it also minimizes the concomitant deterioration of the electrical characteristics of the electrode-tissue interface. The incorporation of a compound such as a steroid that elutes at the site of implantation permits pacing leads to have a source impedance substantially lower as compared to leads featuring similarly sized solid electrodes. Consequently, electronic leads which can elute compounds such as steroids also present significantly lower peak and chronic pacing thresholds than similarly sized electrodes and have therefore been adapted for patient treatment in a variety of contexts.
Implantable compositions which elute a steroid can include a drug blended with a polymeric material such as dexamethasone impregnated within a silicone polymer, a blended composition that is designed to slowly elute the steroid out of the polymer and into the surrounding tissue. Incorporating a drug such as a steroid into a device so that it will elute from a device upon implantation, however, increases the complexity of electronic device production as compared to non-steroid eluting devices. One potential area of difficulty in this context is the possibility of variable manufacturing processes and the potential associated effects on elution kinetics. In this context, methods and materials that allow artisans to readily examine the drug elution properties of electronic devices and other drug eluting medical devices are highly desirable. Such methods and materials can be used for example to assess manufacturing process variability of drug eluting implants and the associated quality control of such processes. Moreover, while real time in vivo elution studies may be necessary to gain a comprehensive mechanistic understanding of the modulation of the physiological reactions observed with implantation, such real time elution tests can be on the order of weeks or months. Consequently, accelerated in-vitro tests that correlate with such tests are important for manufacturing and quality control processes. For this reason, methods and materials such as media and apparatuses that can be used to assess and control the manufacturing process variability of drug eluting implantable devices are highly desirable.